A medium discrimination device for an image-forming apparatus is known in the art. The image-forming apparatus includes an accommodating unit for accommodating a plurality of overlaid sheets and conveys these sheets one at a time from the accommodating unit onto a conveying path. The medium discrimination device comprises transmitted light quantity measuring means disposed along the conveying path that measures the quantity of light transmitted in the thickness direction of the medium to detect overlapped feeds. The medium discrimination device detects the type of medium when the first sheet is conveyed and performs overlapped-feed detection on each sheet thereafter.